Dispatcher UID replaced by a Framework UID

Each framework instance has its own UID, generated using uuid.uuid4() or
given in the framework initialization properties.
The property name is defined in pelix.framework.FRAMEWORK_UID.

The remote service discovery implementations must use this UID in order
to identify a service exporter.
The remote service dispatcher doesn't need a UID anymore.
Bugfix: Multithreaded RPC

There must be at least one remote service proxy per thread in order to
avoid "CannotSendRequest" exceptions.
To simplify the implementation, a ServerProxy is created for each call
to __getattr__.
A future implementation is needed to create as few proxies as possible.

More information:
- http://bugs.python.org/issue14301
- http://stackoverflow.com/questions/13534251/python-httplib-cannotsendrequest-when-nesting-threaded-simplexmlrpcservers
Imported service properties updated

Replacement of the dispatcher UID by the framework UID
EventAdmin: Framework UID in the properties

The UID of the framework sending the event is stored in the event
properties.
This allows to notify remote EventHandler services and let them have
information on the event source.